Crews are still collecting debris and investigating what happened early Sunday morning when a Manhattan-bound train from New York's Metro-North railroad derailed on a sharp turn where operators must decelerate from 70 mph to 30 mph. In a brief press conference Monday afternoon, the National Transport Safety Board (NTSB) reported that the train was traveling at 82 mph going into the turn, well above the safety standard before and during the turn. They've also preliminarily ruled out brake malfunction and signal malfunction, leaving only two possibilities: locomotive error or human error.

NTSB is currently analyzing the train's black box in Washington D.C., which is revealing the timeline of the tragedy. But the agency still doesn't know exactly why operator William Rockefeller dumped the brakes 5 seconds before the train came to rest after derailing.

Dumping the brakes is also known as "going into emergency," says Dave Rangel, a deputy director of the Modoc Railroad Academy. Unlike a car, the brakes on a train are always on and pressurized, releasing compressed air through the brake line is what applies or releases the brake. But when an operator dumps the brakes in an emergency, all air is normalized to atmosphere and the wheels seize up. "An emergency application of the brakes means the train is just sliding," Rangel says. "The engineer has no control of the train." It's a quick but dangerous way to stop a train. PopMech looked into how the almost fail-proof air brakes work after the tragic freight train collision in Quebec earlier this summer.

Despite air brakes being perceived as a fail-safe , another safety net could have helped avoid the crash altogether. After a collision in the Chatsworth district of Los Angeles killed 25 people in 2008, lawmakers began having discussions about positive train control (PTC), a backup system that can automatically wrench controls from an engineer under perceived negligence or emergency. "PTC isn't in effect right now because there are competing systems and no standardization," Rangel says. Metropolitan Transport Authority told The New York Times that it is working to implement a system on their trains, but has yet to do so. When questioned about whether a PTC system could have prevented the Hudson line accident, NTSB officials asserted that right now it's uncertain but urged the Metropolitan Transport Authority (MTA) to adopt any technology that could enhance safety and passenger peace-of-mind.

The derailed train also operated on a push/pull system. When headed northbound, the train was pulled by a locomotive, and when heading southbound it was pushed from behind. Although the 2006 Glendale accident in LA caused officials to question if pushing a locomotive from behind, especially during head-on collisions, was more dangerous than being pulled, the NTSB has said that in this instance, the push/pull system is not the issue, citing the system having navigated the turn without incident until now.

The NTSB is expected to stay at the scene of the accident in the Bronx for at least one more week. The agency has begun interviewing the operator, who was injured in the crash, and have also conducted a toxicology report though no results have been released. The cars and locomotive will soon be moved to a secure location where it will be studied further. "We've got to find out what it is," Rangel says. "There are a lot of trains running out there, and if there's a fatal flaw, we need to find it."